Secondary battery and method for manufacturing same

ABSTRACT

An object of the present invention is to provide a high-quality secondary battery having high electric characteristics and high reliability, the secondary battery preventing a shortcut between a positive electrode and a negative electrode by means of an insulating material and preventing or reducing an increase in volume and deformation of a battery electrode assembly, and a method for manufacturing the same. Secondary battery  100  according to the present invention includes a battery electrode assembly including positive electrode  1  and negative electrode  6  alternately stacked via separator  20.  Positive electrode  1  and negative electrode  2  each includes current collector  3  or  8  and active material  2  or  7  applied to current collector  3  or  8.  Active material  2 A positioned on one surface of positive electrode  1  of current collector  3  includes flat portion  2 A 1  and small-thickness portion (thin-layer portion)  2 A 3  positioned on the end portion side relative to flat portion  2 A 1 , the small-thickness portion  2 A 3  having a thickness that is smaller than that of flat portion  2 A 1 . A portion of active material  2 B positioned on another surface of current collector  3  of positive electrode  1,  the portion facing thin-layer portion  2 A 3  of active material  2 A positioned on the one surface via current collector  3  is a flat portion having a constant thickness.

TECHNICAL FIELD

The present invention relates to a secondary battery including apositive electrode and a negative electrode laid over each other with aseparator interposed therebetween, and a method for manufacturing thesame.

BACKGROUND ART

Secondary batteries are becoming widely used as power supplies forvehicles and household appliances, and not only as power supplies forportable devices such as mobile phones, digital cameras and laptopcomputers, and among others, lithium ion secondary batteries, which havea high-energy density and are lightweight, are energy storage devicesthat are indispensable in daily life.

Secondary batteries are generally classified into a rolled type and astacked type. A battery electrode assembly of a rolled type secondarybattery has a structure in which a long positive electrode sheet and along negative electrode sheet laid on each other via a separator arerolled a plurality of turns. A battery electrode assembly of a stackedtype secondary battery has a structure in which positive electrodesheets and negative electrode sheets are alternately stacked withseparators interposed therebetween. The positive electrode sheets andthe negative electrode sheets each include a current collector includinga coated portion to which active material (which may be a compound agentcontaining, e.g., a binder and a conductive material) has been appliedand an non-coated portion to which active material has not been appliedin order to allow an electrode terminal to be connected thereto.

In either a rolled type secondary battery or a stacked type secondarybattery, a battery electrode assembly is enclosed inside an outercontainer in such a manner that: one end of a positive electrodeterminal is electrically connected to an non-coated portion of apositive electrode sheet and another end of the positive electrodeterminal extends to the outside of the outer container (outer case); andone end of a negative electrode terminal is electrically connected to annon-coated portion of a negative electrode sheet and another end of thenegative electrode terminal extends to the outside of the outercontainer. Inside the outer container, in addition to the batteryelectrode assembly, electrolyte is enclosed. Capacities of secondarybatteries have been increasing year by year, and along with thisincrease, heat that would be generated if a shortcut occurs alsoincreases, causing an increase in risk, and thus, measures to ensurebattery safety are becoming increasingly important.

As an example of a safety countermeasure, a technique in whichinsulating material is formed on a boundary portion between a coatedportion and an non-coated portion in order to prevent a shortcut betweena positive electrode and a negative electrode is known (Patent Document1).

RELATED ART DOCUMENT Patent Document

Patent Document 1: JP2012-164470A

SUMMARY OF INVENTION Problems to be Solved by the Invention

In the technique disclosed in Patent Document 1, as illustrated in FIG.19, positive electrode 1 and negative electrode 6 are alternatelystacked via separator 20, and on current collector 3 of each positiveelectrode 1, insulating material 40 covering boundary portion 4 betweena coated portion to which active material 2 has been applied and annon-coated portion to which active material 2 has not been applied isformed. In a stacked type secondary battery, insulating materials 40 arestacked at the same position in a planar view. Thus, the thickness of aportion of the battery electrode assembly at the position whereinsulating materials 40 are disposed, becomes thicker, which results inreducing energy density per unit volume.

Also, in order to obtain stable electric characteristics and highreliability, it is preferable that the battery electrode assembly of asecondary battery be fastened via, e.g., a tape by applying pressureuniformly. However, use of insulating materials in a stacked typesecondary battery in such a manner as in Patent Document 1 results infailure to uniformly fasten a battery electrode assembly due to adifference in thickness between a portion in which insulating materials40 are present and a portion in which insulating materials 40 are notpresent, which may cause battery quality deterioration such asvariability in electric characteristics and/or degradation of batterycycle properties.

Therefore, an object of the present invention is to solve theaforementioned problems and provide a high-quality secondary batteryhaving high electric characteristics and high reliability, the secondarybattery preventing a short circuit between a positive electrode and anegative electrode by means of insulating material and preventing orreducing an increase in volume and deformation of a battery electrodeassembly, and a method for manufacturing the same.

Means to Solve the Problems

A secondary battery according to the present invention comprises abattery electrode assembly including a positive electrode and a negativeelectrode alternately stacked via a separator, and the positiveelectrode and the negative electrode each includes a current collectorand active material applied to the current collector. The activematerial positioned on one surface of the current collector of thepositive electrode, includes a flat portion and a small-thicknessportion positioned on an end portion side relative to the flat portion,the small-thickness portion having a thickness that is smaller than thatof the flat portion. A portion of the active material positioned onanother surface of the current collector of the positive electrode,which faces the small-thickness portion of the active materialpositioned on the one surface, across the current collector, is a flatportion having a constant thickness.

Advantageous Effect of Invention

The present invention enables preventing or reducing an increase involume of a battery electrode assembly and distortion of the batteryelectrode assembly that are caused by insulating material, and enablesprovision of a high-quality secondary battery having good energydensity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a plan view illustrating a basic structure of a stacked typesecondary battery according to the present invention.

FIG. 1B is a cross-sectional view along line A-A in FIG. 1A.

FIG. 2 is an enlarged cross-sectional view illustrating a positiveelectrode in an exemplary embodiment of a secondary battery of thepresent invention.

FIG. 3 is an enlarged cross-sectional view illustrating a major portionof an exemplary embodiment of a secondary battery according to thepresent invention.

FIG. 4 is an enlarged cross-sectional view illustrating a variation of apositive electrode in an exemplary embodiment of a secondary battery ofthe present invention.

FIG. 5A is a schematic diagram illustrating an example of an electrodecoating apparatus.

FIG. 5B is a schematic diagram illustrating a reference example of anelectrode manufacturing method.

FIG. 6 is a schematic diagram illustrating an example of an electrodemanufacturing method according to the present invention.

FIG. 7 is a plan view illustrating a positive electrode forming step ina secondary battery manufacturing method according to the presentinvention.

FIG. 8 is a plan view illustrating a step following FIG. 7 in thesecondary battery manufacturing method according to the presentinvention.

FIG. 9A is a plan view illustrating a step following FIG. 8 in thesecondary battery manufacturing method according to the presentinvention.

FIG. 9B is a plan view illustrating a positive electrode formed as aresult of cutting in the step illustrated in FIG. 9A.

FIG. 10 is a plan view illustrating a negative electrode forming step inthe secondary battery manufacturing method according to the presentinvention.

FIG. 11A is a plan view illustrating a step following FIG. 10 in thesecondary battery manufacturing method according to the presentinvention.

FIG. 11B is a plan view illustrating a negative electrode formed as aresult of cutting in the step illustrated in FIG. 11A.

FIG. 12 is an enlarged cross-sectional view illustrating a major portionof another exemplary embodiment of a secondary battery according to thepresent invention.

FIG. 13 is a block diagram schematically illustrating an example of anapparatus used for intermittent application of active material.

FIG. 14A is a cross-sectional view schematically illustrating an exampleof an apparatus used for continuous application of active material.

FIG. 14B is an enlarged cross-sectional view along line B-B in FIG. 14B.

FIG. 15 is a plan view illustrating another example of a positiveelectrode forming step in a secondary battery manufacturing methodaccording to the present invention.

FIG. 16 is a plan view illustrating a step following FIG. 15 in thesecondary battery manufacturing method according to the presentinvention.

FIG. 17A is a plan view illustrating a step following FIG. 16 in thesecondary battery manufacturing method according to the presentinvention.

FIG. 17B is a plan view illustrating a positive electrode formed as aresult of cutting in the step illustrated in FIG. 17A.

FIG. 18 is a perspective diagram illustrating an electrode roll used inthe secondary battery manufacturing method illustrated in FIGS. 15 to17B.

FIG. 19 is an enlarged view illustrating a major portion of a stackedtype secondary battery according to a related art.

DESCRIPTION OF EXEMPLARY EMBODIMENT

An exemplary embodiment of the present invention will be described belowwith reference to the drawings.

Basic Configuration of Secondary Battery

FIG. 1 schematically illustrates an example of a configuration of astacked type lithium ion secondary battery employing the presentinvention. FIG. 1A is a plan view as viewed from the upper sideperpendicular to a principal surface (planar surface) of the secondarybattery, and FIG. 1B is a cross-sectional view along line A-A in FIG.1A.

Lithium ion secondary battery 100 according to the present inventionincludes an electrode stack (battery electrode assembly) formed byalternately stacking positive electrodes (positive electrode sheets) 1and negative electrodes (negative electrode sheets) 6 via separators 20.The electrode stack is housed together with an electrolyte in an outercontainer consisting of flexible films 30. One end of positive electrodeterminal 11 is connected to positive electrodes 1 of the electrodestack, and one end of negative electrode terminal 16 is connected tonegative electrodes 6, and another end side of positive electrodeterminal 11 and another end side of negative electrode terminal 16extend to the outside of the flexible films 30. In FIG. 1B, illustrationof a portion of the layers (layers positioned in the intermediateportion in a thickness direction) included in the electrode stack isomitted but the electrolyte is illustrated.

Each positive electrode 1 includes positive-electrode current collector3 and positive-electrode active materials 2 applied topositive-electrode current collector 3, and on each of a front surfaceand a back surface of positive-electrode current collector 3, a coatedportion to which positive-electrode active material 2 has been appliedand an non-coated portion to which positive-electrode active material 2has not been applied are positioned side by side along a longitudinaldirection. Likewise, each negative electrode 6 includesnegative-electrode current collector 8 and negative-electrode activematerials 7 applied to negative-electrode current collector 8, and oneach of a front surface and a back surface of negative-electrode currentcollector 8, a coated portion and an non-coated portion are positionedside by side along the longitudinal direction. A planar position ofboundary portion 4 between the coated portion and the non-coated portionof each positive electrode 1 and a planar position of boundary portion 4between the coated portion and the non-coated portion of each negativeelectrode 6 may be the same or different (not aligned in planar view)between the front surface and the back surface of the relevant currentcollector.

The non-coated portion of each of positive electrodes 1 and negativeelectrodes 6 is used as a tab for connection with an electrode terminal(positive electrode terminal 11 or negative electrode terminal 16). Thepositive electrode tabs connected to respective positive electrodes 1are bundled on positive electrode terminal 11 and are mutually connectedtogether with positive electrode terminal 11 by means of, e.g.ultrasonic welding. The negative electrode tabs connected to respectivenegative electrodes 6 are bundled on negative electrode terminal 16 andare mutually connected together with negative electrode terminal 16 bymeans of, e.g., ultrasonic welding. On that basis, the other end portionof positive electrode terminal 11 and the other end portion of negativeelectrode terminal 16 extend to the outside of the outer container.

As illustrated in FIG. 2, insulating material 40 for preventing theoccurrence of a short circuit in negative electrode terminal 16 isformed so as to cover boundary portion 4 between the coated portion andthe non-coated portion of each positive electrode 1. Insulating material40 is preferably formed to straddle both the positive electrode tab andpositive-electrode active material 2 so as to cover boundary portion 4.Formation of insulating material 40 will be described later.

Outer dimensions of the coated portion (negative-electrode activematerial 7) of each negative electrode 6 are larger than those of thecoated portion (positive-electrode active material 2) of each positiveelectrode 1 and are smaller than or equal to those of each separator 20.

In the battery illustrated in FIGS. 1A and 1B, examples ofpositive-electrode active material 2 include layered oxide materialssuch as LiCoO₂, LiNiO₂, LiNi_((1-x))CoO₂, LiNi_(x)(CoAl)_((1-x))O₂,Li₂MO₃—LiMO₂, LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, spinel materials such asLiMn₂O₄, LiMn_(1.5)Ni_(0.5)O₄, LiMn_((2-x))M_(x)O₄, olivine materialssuch as LiMPO₄, olivine fluoride materials such as Li₂MPO₄F andLi₂MSiO₄F, and vanadium oxide materials such as V₂O₅. One of the abovematerials or a mixture of two or more selected from among the abovematerials may be used as positive-electrode active material 2.

As negative-electrode active material 7, carbon materials such asgraphite, amorphous carbon, diamond-like carbon, fullerene, carbonnanotube, carbon nanohorn, lithium metal materials, silicon- ortin-based alloy materials, oxide-based materials such as Nb₂O₅ and TiO₂,or a composite of them may be used.

A binding agent and/or a conductive assistant may arbitrarily be addedto positive-electrode active material 2 and negative-electrode activematerial 7. As the conductive assistant, carbon black or carbon fiber orgraphite or the like can be used and the combination of two or more ofthe above materials can be used. As the binding agent, polyvinylidenefluoride, polytetrafluoroethylene, carboxymethyl cellulose, modifiedacrylonitrile rubber particles or the like may be used.

As positive-electrode current collector 3, aluminum, stainless steel,nickel, titanium or an alloy containing any of these materials can beused, and in particular, aluminum is preferable. As negative-electrodecurrent collector 8, copper, stainless steel, nickel, titanium or analloy containing any of these materials can be used.

As the electrolyte, one organic solvent selected from among cycliccarbonates such as ethylene carbonate, propylene carbonate, vinylenecarbonate and butylene carbonate, chain carbonates such as ethyl methylcarbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC) anddipropyl carbonate (DPC), aliphatic carboxylic acid esters, γ-lactonessuch as γ-butyrolactone, chain ethers, and cyclic ethers may be used andthe mixture of two or more of the above materials may be used.Furthermore, a lithium salt can be dissolved in the organic solvent(s).

Separator 20 is formed mainly of a porous membrane, woven fabric,nonwoven fabric that are made of resin. As the resin component inseparator 20, polyolefin-based resin such as polypropylene orpolyethylene, polyester resin, acrylic resin, styrene resin, nylon resinor the like can be used, for example. A polyolefin-based microporousmembrane is particularly preferable because the polyolefin-basedmicroporous membrane has excellent ion-permeating properties andexcellent performance characteristics for physically separating apositive electrode and a negative electrode. A layer containinginorganic particles may be formed in separator 20 as needed. Examples ofthe inorganic particles include insulating oxide, silicate, nitride, andcarbide. In particular, the inorganic particles preferably contain TiO₂or Al₂O₃.

As the outer container, a case made of flexible film 30 or a can casecan be used. From the point of view of battery weight reduction, usingflexible film 30 is preferable. As flexible film 30, a film in whichresin layers are provided on both the front and rear surfaces of a metallayer as a base material can be used. As the metal layer, a layer havingbarrier properties which may be properties for preventing leakage of anelectrolyte and infiltration of moisture from the outside can beselected, and aluminum, stainless steel or the like can be used. Athermally-fusible resin layer such as modified polyolefin is provided onat least one surface of the metal layer. The thermally-fusible resinlayers of flexible film 30 are opposite to each other and are thermallyfused to each other in the portion that surrounds the space where thelaminated electrode assembly is stored, thereby the outer container isformed. A resin layer such as a nylon film and a polyester film may beprovided on the surface of the outer container opposite to the surfaceon which the thermally-fusible resin layer is formed.

As positive electrode terminal 11, a terminal formed of aluminum oraluminum alloy can be used. As negative electrode terminal 16, aterminal formed of copper, copper alloy, or nickel-plated copper orcopper alloy can be used. Each of the other ends of terminals 11 and 16extends to the outside of the outer container. Thermally-fusible resincan be provided in advance at each of the positions of terminals 11 and16 corresponding to the thermal-welded portions of the outer peripheralof the outer container.

Insulating material 40 that is formed to cover boundary portion 4 abetween a coated portion and an non-coated portion of positive electrodeactive material 2 can be made of polyimide, glass fiber, polyester,polypropylene, or a material including these. Insulating material 40 maybe formed by applying heat to a tape-like resin member to weld the resinmember to boundary portion 4 a or by applying a gel resin to boundaryportion 4 a and drying the resin.

Detailed Configuration of Electrodes

FIG. 2 is a schematic cross-sectional view for describing an exemplaryembodiment of lithium ion secondary battery 100 according to the presentinvention, and schematically illustrates only a portion of an electrodestack in an enlarged manner. Here, a portion around an end portion onthe positive electrode tab side of positive-electrode active material 2is illustrated. FIG. 3 illustrates an electrode stack including positiveelectrodes 1.

As illustrated in FIGS. 2 and 3, positive-electrode active material 2 isformed on each of opposite surfaces of each positive-electrode currentcollector 3, and although not illustrated in FIGS. 1A and 1B, insulatingmaterial 40 is provided to straddle a coated portion to whichpositive-electrode active material 2 has been applied and an non-coatedportion (positive electrode tab) to which positive-electrode activematerial 2 has not been applied. First positive-electrode activematerial layer 2A formed on one surface (upper surface in FIG. 2) ofeach positive-electrode current collector 3 includes flat portion 2A₁,inclined portion 2A₂ and thin-layer portion 2A₃. Thin-layer portion 2A₃is a portion positioned on the end portion side (positive electrode tabside) relative to the flat portion 2A₁, the portion having a thicknessthat is smaller than that of flat portion 2A₁. Inclined portion 2A₂ is aportion whose thickness continuously decreases so as to smoothly connectthick flat portion 2A₁ and thin-layer portion 2A₃. However, instead ofinclined portion 2A₂, a stepped portion whose thickness intermittentlydecreases may be provided. On the other hand, second positive-electrodeactive material layer 2B formed on another surface (lower surface inFIG. 2) of each positive-electrode current collector 3 includes only aflat portion. One end portion 40 a of insulating material 40 ispositioned on thin-layer portion 2A₃ of first positive-electrode activematerial layer 2A, and another end portion 40 b is positioned on thenon-coated portion, that is, the portion of positive-electrode currentcollector 3 on which positive-electrode active material 2 is not formed(positive electrode tab). As illustrated in FIG. 3, in each negativeelectrode 6, also, negative-electrode active material 7 is applied toeach of a front surface and a back surface of each negative-electrodecurrent collector 8; however, negative-electrode active material 7includes only a flat portion, and includes neither an inclined portionnor a thin-layer portion.

The difference in thickness between flat portion 2A₁ and thin-layerportion 2A₃ of first positive-electrode active material 2A is preferablylarger than the thickness of insulating material 40. Also, end portion40 a of insulating material 40 positioned on second positive-electrodeactive material layer 2B is preferably positioned so as to facethin-layer portion 2A₃ of first positive-electrode active material layer2A. Such disposition enables preventing or reducing an increase inthickness caused by insulating materials 40 positioned on the oppositesurfaces of each positive-electrode current collector 3. In other words,adjustment (reduction) of the thickness of an outer edge portion of eachfirst positive-electrode active material layer 2A (coated portion)enables preventing or reducing an increase in thickness in the portionof the electrode stack in which insulating materials 40 are positioned,thereby preventing characteristics of the battery from being affected bythe thickness increase. In particular, if the difference in thicknessbetween thin-layer portion 2A₃ and flat portion 2A₁ of each firstpositive-electrode active material layer 2A is no less than twice thethickness of one insulating material 40, such a degree of difference iseffective because an increase in thickness caused by two insulatingmaterials 40 can be absorbed by the thickness reduction provided bythin-layer portion 2A₃ of first positive-electrode active material layer2A. Here, since it is not necessary that positive-electrode activematerial layers 2A and 2B on opposite surface of positive-electrodecurrent collector 3 have the same thickness, and thus, even where thethickness of one positive-electrode active material layer (secondpositive-electrode active material layer 2B) is less than twice thethickness of insulating material 40, as illustrated in FIG. 4, if onlyflat portion 2A₁ of another positive-electrode active material layer(first positive-electrode active material layer 2A) is made thicker inorder that a difference in thickness between flat portion 2A₁ andthin-layer portion 2A₃ may be no less than twice the thickness ofinsulating material 40, the thickness increase that is caused by twoinsulating materials 40 can be absorbed by reduction in the thicknessprovided by thin-layer portion 2A₃, and thus a sufficient effect can beprovided.

At an end portion of each negative electrode 6 on the side that is thesame as the end portion of each first positive-electrode active materiallayer 2A in which inclined portion 2A₂ and thin-layer portion 2A₃ areprovided as described above, negative-electrode current collector 8 andflat negative-electrode active materials 7 formed on opposite surfacesthereof are cut and terminated. In other words, at the end portion ofeach negative-electrode active material 7 on the side that is the sameas the side of each first positive-electrode active material layer 2A inwhich inclined portion 2A₂ and thin-layer portion 2A₃ are provided,neither an inclined portion, nor a stepped portion nor a thin-layerportion is provided. The end portion is located at a position facingrelevant insulating material 40 across relevant separator 20.

In FIG. 3, for ease of viewing, positive electrodes 1, negativeelectrode 6 and separators 20 are illustrated so as not to be in contactwith one another; however, in reality, positive electrodes 1, negativeelectrode 6 and separators 20 are stacked in close contact with oneanother. In the configuration illustrated in FIG. 3, as described above,the difference in thickness between inclined portion 2A₁ and thin-layerportion 2A₃ of each first positive-electrode active material layer 2A ismore than twice the thickness of each insulating material 40, and thus,when positive electrodes 1, negative electrodes 6 and separators 20 arebrought into close contact with one another, positive electrodes 1 arecurved at respective positions of thin-layer portions 2A₃, enablingpreventing or reducing a partial increase in the thickness of theelectrode stack caused by insulating materials 40. As described above,FIG. 3 illustrates a configuration in which positive electrodes 1 arecurved; however, a configuration in which only negative electrodes 6 arecurved or a configuration in which both positive electrodes 1 andnegative electrodes 6 are curved can be employed.

Here, it is not necessary that flat portion 2A₁ and thin-layer portion2A₃ be disposed in parallel to each other on each positive-electrodecurrent collector 3, and an edge of boundary portion 4 between a coatedportion and an non-coated portion of each positive electrode 1 and anedge of an end portion of each negative electrode 6 may each have around curve shape, rather than a linear shape perpendicular to adirection in which relevant current collector 3 or 8 extends. It shouldbe understood that each of positive-electrode active materials 2 andnegative-electrode active materials 7 may include e.g., an unavoidableinclination, irregularities or roundness of respective layers due to,for example, manufacturing variations and/or layer formation capability.

Each first positive-electrode active material layer 2A may include astepped portion whose thickness decreases in a stepwise fashion, insteadof inclined portion 2A₂ whose thickness gently decreases as illustratedin FIG. 3. Alternatively, each first positive-electrode active materiallayer 2A may include both inclined portion 2A₂ and the stepped portion.Also, it is possible that: thin-layer portion 2A₃ is not providedindependently from inclined portion 2A₂ and the stepped portion; and aportion of inclined portion 2A₂ or the stepped portion which has adecreased thickness, faces relevant insulating material 40, whereby thethickness increase caused by insulating material 40 is absorbed. In sucha case, the portion of the inclined portion 2A₂ or the stepped portionwhich faces insulating material 40, can be regarded as acting asthin-layer portion 2A₃. Inclined portions 2A₂ and thin-layer portions2A₃ illustrated in FIGS. 2 to 4 and the non-illustrated stepped portionseach have a low density compared to flat portions 2A₁.

In the configuration illustrated in FIG. 3, inclined portion 2A₂ andthin-layer portion 2A₃ are formed only in each first positive-electrodeactive material layer 2A, rather than an inclined portion or a steppedportion and a thin-layer portion being provided in each of bothpositive-electrode active material layers 2A and 2B, mainly because theshape of thin-layer portion 2A₃ can be formed with good precision andbecause the electrode capacity loss is small.

For example, if an inclined portion, a stepped portion and/or athin-layer portion are provided in each of both first positive-electrodeactive material layers 2A and second positive-electrode active materials2B, and if insulating material 40 is disposed so as to face the inclinedportion, the stepped portion and/or the thin-layer portion enablespreventing or reducing a partial increase in thickness caused byinsulating materials. However, a thickness reduction causes a reductionin the amount of active materials, which results in a decrease inbattery capacity. Also, the inventors' careful studies revealed thatprovision of thin-layer portion 2A₃ in each of positive-electrode activematerial layers 2A and 2B may make it impossible for thin-layer portion2A₃ to have a sufficiently-small thickness. In such a case, theelectrodes could not be used as products and would be discarded asdefective products, which results in deterioration in productivity.Also, provision of a thin-layer portion, an inclined portion and/or astepped portion in each negative-electrode active material 8 of eachnegative electrode 6 facing relevant positive electrode 1 acrossrelevant separator 20 has the effect of preventing or reducing a partialthickness increase caused by insulating materials 40; however, in such acase, the amount of negative-electrode active materials 8 decreases,also unfavorably resulting in a battery capacity decrease.

For a more detailed evaluation, it was found that a failure to form athin-layer portion, an inclined portion and/or a stepped portion ofpositive-electrode active material 2 with good precision and unstableformation of the thin-layer portion, the inclined portion and/or thestepped portion are partly attributable to a tendency of such portionsbeing formed so as to lean toward either first positive-electrode activematerial 2A or second positive-electrode active material 2B. This willbe described using the reference example illustrated in FIGS. 5A and 5B.

FIG. 5A is a schematic diagram indicating a coating portion of a diecoater, which is a kind of apparatuses for coating electrodes. The diecoater applies slurry 200 to a current collector between die head 500and back roll 400. Slurry 200 containing an active material isdischarged from discharge port 501 of die head 500 toward the currentcollector transported on the outer peripheral surface of back roll 400.The thickness of slurry 200 on the current collector is controlled byadjusting, e.g., a space between the current collector and dischargeport 501, the discharge amount and/or the application speed accordingto, e.g., a viscosity of slurry 200. In the example illustrated in FIGS.5A and 5B, slurry 200 containing positive-electrode active material 2 isintermittently applied to positive-electrode current collector 3. Itshould be understood that slurry 200 can continuously be applied topositive-electrode current collector 3.

FIG. 5B illustrates a state in which after application of firstpositive-electrode active material layer 2A to one surface ofpositive-electrode current collector 3 and after drying of firstpositive-electrode active material layer 2A, second positive-electrodeactive material 2B is applied to another surface of positive-electrodecurrent collector 3. Each of first positive-electrode active materiallayer 2A and second positive-electrode active material layer 2B isintermittently formed, and an inclined portion and a thin-layer portionare formed at each opposite end (an application start end and anapplication termination end) of each coated portion. When slurry 200 isdischarged from discharge port 501 of die head 500 in order to formsecond positive-electrode active material layer 2B on the surface ofpositive-electrode current collector 3 on the side opposite to a surfaceon which first positive-electrode active material layer 2A has alreadybeen formed, a gap is generated between the inclined portion and thethin-layer portion of each first positive-electrode active material 2Aand back roll 400. Slurry 200 is pressurized in die head 500, and upondischarge of slurry 200, positive-electrode current collector 3 ispushed in a direction in which gap h is eliminated, that is, toward theback roll 400 side, whereby the space between the discharge port 501 andpositive-electrode current collector 3 is increased. As described above,it was found that if an active material including an inclined portionand a thin-layer portion is formed on one surface and then an activematerial is formed on another surface, the space between discharge port501 and the current collector is not stable and an active material thatis subsequently formed tends to have an unstable thickness andinclination.

Therefore, in the present invention, as illustrated in FIG. 6, afterflat second positive-electrode active material layer 2B, that does notinclude a thin-layer portion, an inclined portion, and a stepped portionare formed on positive-electrode current collector 3; firstpositive-electrode active material layer 2A, that includes thin-layerportion 2A₃ and inclined portion 2A₂ are formed on a surface opposite tothe surface on which positive-electrode active material layer 2B hasbeen formed. Then, the portion of positive-electrode current collector3, to which first positive-electrode active material layer 2A will besubsequently applied, is a portion on the opposite side of a portion inwhich flat second positive-electrode active material layer 2B comes intoclose contact with back roll 400 with no gap therebetween. Since no gapis generated between second positive-electrode active material 2B andback roll 400, when first positive-electrode active material 2A isformed on positive-electrode current collector 3, the space between thedischarge port 501 and positive-electrode current collector 3 isextremely stable, enabling formation of inclined portion 2A₂ andthin-layer portion 2A₃ with very good precision. Therefore, thedifference in thickness between flat portion 2A₁ and thin-layer portion2A₃ of first positive-electrode active material layer 2A can be made tobe no less than twice the thickness of insulating material 40 with goodprecision. Even if the thickness of positive-electrode active material 2is so small such that a thin-layer portion having a thickness decreasedby no less than twice the thickness of the insulating material cannot beformed and such that a thickness increase caused by insulating materials40 cannot completely be absorbed by first positive-electrode activematerial layer 2A alone, the thickness of first positive-electrodeactive material 2A can be controlled with good precision, enablingpreventing or reducing a partial increase in the thickness of theelectrode stack by reducing a thickness of either or both of thenegative-electrode active materials to the minimum necessary atrespective positions facing the insulating materials.

As described above, provision of a thin-layer portion, an inclinedportion and/or a stepped portion in the active material provided on onesurface of a current collector effectively prevents or reduces a partialincrease in the thickness at a position where insulating materials areprovided, and in addition, providing neither a thin-layer portion, noran inclined portion, nor a stepped portion in the active materialprovided on another surface of the current collector enablesproductivity enhancement.

Electrode Manufacturing Method

First, as described above, in the step illustrated in FIG. 6,positive-electrode active material 2 is intermittently applied to eachof the opposite surfaces of long band-like positive-electrode currentcollector 3 for manufacturing a plurality of positive electrodes(positive electrode sheets) 1. In FIG. 7, a surface on the firstpositive-electrode active material layer 2A side of positive-electrodecurrent collector 3 with positive-electrode active material 2 applied toeach of the opposite surfaces thereof is illustrated. Although notclearly illustrated in FIG. 7, each first positive-electrode activematerial layer 2A includes inclined portion 2A₂ and thin-layer portion2A₃ in the vicinity of boundary portion 4, which serves as a positiveelectrode tab. Then, as illustrated in FIG. 8, insulating material 40 isformed so as to cover boundary portion 4. As illustrated in FIGS. 2 and3, one end portion 40 a of insulating material 40 is positioned onthin-layer portion 2A₃, and another end portion 40 b of insulatingmaterial 40 is positioned on an non-coated portion. If the thickness ofinsulating material 40 is too small, a sufficient insulating propertycannot be ensured and thus the thickness is preferably no less than 10μm. Also, if the thickness of the insulating material 40 is excessivelylarge, the effect of preventing or reducing an increase in the thicknessof the electrode stack, which is provided by the present invention,cannot be sufficiently realized, and thus, insulating material 40 ispreferably smaller in thickness than the flat portion ofpositive-electrode active material 2. The thickness of insulatingmaterial 40 is preferably no more than 90% of the thickness of the flatportion of positive-electrode active material 2, more preferably no morethan 60% of the thickness of flat portion 2 b. Although the end portionof each coated portion (positive-electrode active material 2) atboundary portion 4 between the coated portion and the relevantnon-coated portion may rise substantially perpendicularly to relevantpositive-electrode current collector 3 as illustrated in FIGS. 2 to 4,the end portion may be slightly inclined as illustrated in FIG. 19.Also, in each negative electrode 6, the end portion of each coatedportion (negative-electrode active material 7) may be slightly inclinedor rise substantially perpendicular to relevant negative-electrodecurrent collector 8.

Subsequently, in order to obtain positive electrodes 1 used forindividual stacked type batteries, positive-electrode current collector3 is cut along each cutting line 90 indicated by a dashed line in FIG.9A to obtain positive electrodes 1 of a desired size, one of which isillustrated in FIG. 9B. The cutting lines 90 are imaginary lines andthus not actually formed.

Meanwhile, with a method that is similar to the step illustrated in FIG.6, negative-electrode active material 7 is intermittently applied toeach of the opposite surfaces of large negative-electrode currentcollector 8, which is provided for manufacturing a plurality of negativeelectrodes (negative electrode sheets) 6. In FIG. 10, negative-electrodecurrent collector 8 with negative-electrode active material 7 applied oneach of the opposite surfaces thereof is illustrated. If the differencein thickness between flat portion 2A₁ and thin-layer portion 2A₃ of eachfirst positive-electrode active material layer 2A is no less than twicethe thickness of each insulating material 40 as illustrated in FIGS. 2and 3, negative-electrode active material 7 may include a flat portionalone in which neither an inclined portion, nor a thin-layer portion,nor a stepped portion are present.

Subsequently, in order to obtain negative electrodes 6 to be used forindividual stacked type batteries, negative-electrode current collector8 is divided by cutting negative-electrode current collector 8 alongeach cutting line 91 indicated by a dashed line in FIG. 11A to obtainnegative electrodes 6 having a desired size, one of which is illustratedin FIG. 11B. Cutting lines 91 are imaginary lines and thus are notactually formed.

Positive electrodes 1 illustrated in FIG. 9B and negative electrodes 6illustrated in FIG. 11B formed as described above are alternatelystacked via separators 20, and positive electrode terminal 11 andnegative electrode terminal 16 are connected to the stacked electrodes,whereby the electrode stack illustrated in FIG. 3 is formed. Theelectrode stack is housed and sealed together with electrolyte in anouter container including flexible films 30, whereby secondary battery100 illustrated in FIGS. 1A and 1B is formed. In secondary battery 100according to the present invention, which has been formed as describedabove, one end portion 40 a of each insulating material 40 is positionedon thin-layer portion 2A₃ of relevant first positive-electrode activematerial layer 2A.

According to secondary battery 100, the amount of thickness increasecaused by each insulating material 40 formed so as to cover boundaryportion 4 between the coated portions and the non-coated portion ofrelevant positive electrode 1 is absorbed (cancelled out) by thethickness reduction provided by thin-layer portion 2A₃ and inclinedportion 2A₂ of relevant first positive-electrode active material layer2A, preventing or reducing a partial increase in the thickness of theelectrode stack, and thus, the electrode stack can be uniformly fastenedand held in place, thereby preventing or reducing a deterioration inproduct quality as regards, for example, variability in the electriccharacteristics and battery cycle degradation.

In the example illustrated in FIG. 11B, the coated portion at each ofthe opposite surfaces of each negative electrode 6 is cut and terminatedat a position facing the non-coated portion (positive electrode tab) ofrelevant positive electrode 1, and as illustrated in FIG. 3, at aposition facing the non-coated portion of each positive electrode 1,negative-electrode active material 7 exists on the front and back ofnegative-electrode current collector 8 with no non-coated portionprovided. However, each negative electrode 6 may also be configured insuch a manner that an non-coated portion is present at a position innegative electrode 6, the position facing the non-coated portion ofpositive electrode 1. As illustrated in FIG. 11B, at an end portion ofeach negative electrode 6, the end portion not facing the non-coatedportion of relevant positive electrode 1, an non-coated portion, whichserves as a negative electrode tab, is provided. If insulating material(not illustrated) is provided on a boundary portion between the coatedportion and the non-coated portion of each negative electrode 6, as inthe case in which a thickness increase caused by insulating material 40is cancelled out by means of each positive electrode 1, a thin-layerportion, an inclined portion and/or a stepped portion having a smallthickness may be provided in each negative-electrode active material orin each positive-electrode active material, and insulating material maybe disposed at a position facing the thin-layer portion, the inclinedportion and/or the stepped portion.

As illustrated in FIG. 12, an inclined portion 7 a can be provided in atleast one of negative-electrode active materials 7 in each negativeelectrode 6 to further reduce the possibility of battery distortion dueto insulating materials 40 provided on positive electrodes 1. Eachinsulating material 40 with one end portion 40 a positioned onthin-layer portion 2A₃ of relevant first positive-electrode activematerial layer 2A is preferably formed in such a manner that the totalthickness of two insulating materials 40 is no larger than thedifference in thickness between flat portion 2A₁ and thin-layer portion2A₃ of each first positive-electrode active material layer 2A. However,manufacturing variations may prevent the difference in thickness betweenflat portion 2A₁ and thin-layer portion 2A₃ of each firstpositive-electrode active material layer 2A from becoming the desiredsize. Even if such manufacturing variations occur, the presence ofinclined portion 7 a of each negative-electrode active material 7enables the thickness increase caused by manufacturing variations ofpositive electrodes 1 to be absorbed (cancelled out). In FIG. 12, aconfiguration in which inclined portion 7 a of each negative-electrodeactive material 7 is positioned facing insulating material 40 onrelevant first positive-electrode active material layer 2A, whichincludes inclined portion 2A₁ and thin-layer portion 2A₃, acrossrelevant separator 20 is illustrated as an example. However, inclinedportion 7 a may be disposed so as to face insulating material 40 onrelevant second positive-electrode active material layer 2B, which hasneither an inclined portion nor a thin-layer portion, across relevantseparator 20.

Unless otherwise specified, each of thicknesses, distances, etc., of therespective members in the present invention means the average value ofvalues measured at three or more arbitrary positions.

EXAMPLES Example 1

According to the manufacturing method described with reference to FIGS.6 to 12, a lithium ion secondary battery was manufactured.

First, a mixed active material of LiMn₂O₄ andLiNi_(0.8)Co_(0.1)Al_(0.1)O₂ was used as positive-electrode activematerial, carbon black was used as a conductive agent, and PVdF was usedas binder, and slurry 200 in which a compound agent consisting of thesematerials is dispersed in an organic solvent was prepared. This slurry200 was intermittently applied to positive-electrode current collector 3having a thickness of 20 μm and mainly consisting of aluminum and thendried, whereby second positive-electrode active material layers 2Bhaving a thickness of 80 μm was formed. As a result of the intermittentapplication of positive-electrode active material 2, coated portionscoated with positive-electrode active material 2 and non-coated portionsnot coated with positive-electrode active material 2 are alternatelypresent along a longitudinal direction of positive-electrode currentcollector 2. Next, as illustrated in FIGS. 6 and 7, positive-electrodeactive material 2 was intermittently applied to the surface ofpositive-electrode current collector 3 on the side that is opposite tothe side on which second positive-electrode active material layers 2Bwas formed, and then dried, whereby first positive-electrode activematerial layers 2A were formed. Each first positive-electrode activematerial layer 2A was configured so as to include flat portion 2A₁having a thickness of 80 μm, thin-layer portion 2A₃ having a thickness20 μm and inclined portion 2A₂ whose thickness continuously decreasesbetween flat portion 2A₁ and thin-layer portion 2A₃.

A method of applying an active material to a current collector will bedescribed. As an apparatus that applies active material, any device thatutilizes various coating methods including transfer methods or vapordeposition methods, such as doctor blades, die coaters and gravurecoaters, may be used. In the present invention, in order to control theposition of an end portion of applied active material, it isparticularly preferable to use a die coater such as illustrated in FIG.6. Methods of coating active material that use a die coater aregenerally classified into two methods: a continuous application methodin which active material is continuously formed in the longitudinaldirection of a long current collector, and an intermittent coatingmethod in which coated portions coated with active material andnon-coated portions not coated with the active material are alternatelyformed along a longitudinal direction of a current collector.

FIG. 13 is a diagram illustrating an example of a configuration of a diecoater that intermittently applies active material. As illustrated inFIG. 13, a slurry flow path of a die coater that performs intermittentcoating includes die head 500, coating valve 502 connected to die head500, pump 503, and tank 504 that stores slurry 200. Also, return valve505 is provided between tank 504 and coating valve 502. In thisconfiguration, at least for coating valve 502, it is preferable to use amotor valve. A motor valve can vary an open/closed state of the valvewith good precision even while slurry 200 is being applied. Therefore,e.g., the flow path of slurry 200 is controlled by coating valve 502,which includes a motor valve, in combination with the operation ofreturn valve 505, enabling a coated portion (flat portion 2A₁, inclinedportion 2A₂ or a stepped portion and thin-layer portion 2A₃) of eachactive material, an non-coated portion and a boundary portiontherebetween to be formed into respective desired shapes.

Also, active material can be formed by being continuously applied usingthe die coater schematically illustrated in FIGS. 14A and 14B. At eachof opposite end portions of discharge port 501 of die head 500 of thedie coater, shim 501 b including a tapered portion or stepped portion501 a whose thickness decreases toward the center portion of dischargeport 501 is provided. Shims 501 b enable formation of active material insuch a manner that a stepped portion or an inclined portion and athin-layer portion are formed at an end portion of each coated portion.

After the coating positive-electrode active material 2 onpositive-electrode current collector 3 as described above, asillustrated in FIG. 8, polypropylene insulating tape (insulatingmaterials) 40 having a thickness of 30 μm was attached to boundaryportion 4 between the coated portion and the non-coated portion of eachpositive electrode 1. Here, insulating tape 40, provided so as to coverboundary portion 4 on one surface of each positive-electrode activematerial 2, was formed so that end portion 40 a is positioned onthin-layer portion 2A₃ of relevant first positive-electrode activematerial layer 2A. Insulating tape 40, provided so as to cover boundaryportion 4 on another surface of each positive-electrode active material2, was disposed so that one end portion 40 a faces thin-layer portion2A₃ of relevant first positive-electrode active material layer 2A acrosspositive-electrode current collector 3. Then, as illustrated in FIGS. 9Aand 9B, positive-electrode current collector 3 was cut along eachcutting line 90 to obtain individual positive electrodes 1.

<Negative Electrodes>

Graphite with a surface coated with an amorphous material was used asnegative-electrode active material 7 and PVdF was used as a binder, anda slurry in which a compound agent of these materials is dispersed in anorganic solvent was prepared. As illustrated in FIG. 10, the slurry wasintermittently applied to a copper foil having a thickness of 15 μm,which is negative-electrode current collector 8, and then dried tofabricate a negative electrode roll including coated portions coatedwith negative-electrode active material 7 and non-coated portions notcoated with negative-electrode active material 7 as with positiveelectrodes 1. Each negative-electrode active material 7 includes only aflat portion having a thickness of 55 μm. A specific method for applyingnegative-electrode active material 7 is similar to the aforementionedmethod for applying positive-electrode active material 2, and activematerial may be intermittently applied using the die coater illustratedin FIG. 13 or may be continuously applied using the die coaterillustrated in FIGS. 14A and 14B. Then, as illustrated in FIGS. 11A and11B, negative-electrode current collector 8 was cut along each cuttingline 91 to obtain individual negative electrodes 6. Each negativeelectrode 6 includes a negative electrode tab, which is an non-coatedportion not coated by negative-electrode active material 7, at aposition that does not face a positive electrode tab, andnegative-electrode current collector 8 was cut at a portion that faces apositive electrode tab and that has negative-electrode active materials7 on both surfaces thereof. Insulating material is not provided at aboundary portion between the coated portion and the non-coated portionof each negative electrode 6.

<Manufacturing of Stacked Type Battery>

Obtained positive electrodes 1 and negative electrodes 6 werealternately stacked via separators 20 having a thickness of 25 μm, eachseparator 20 made of polypropylene, and negative electrode terminal 16and positive electrode terminal 11 were attached to the stack, which wasthen housed in an outer container consisting of flexible films 30,whereby a stacked type secondary battery having a thickness of 8 mm wasobtained.

Example 2

Using a compound agent containing LiMn₂O₄, which is active material 2,carbon black, which is a conductive agent, and PVdF, which is a binder,positive-electrode active material 2 was formed on each of the oppositesurfaces of positive-electrode current collector 3. Each firstpositive-electrode active material layer 2A according to the presentexample includes a flat portion 2A₁ having a thickness of 35 μm, athin-layer portion 2A₃ having a thickness of 5 μm, and an inclinedportion 2A₂ whose thickness continuously decreases between flat portion2A₁ and thin-layer portion 2A₃. Each second positive-electrode activematerial layer 2B includes only a flat portion having a thickness of 35μm. Next, as in example 1, polypropylene insulating tapes (insulatingmaterials) 40 having a thickness of 30 μm were attached and thenpositive-electrode current collector 3 was cut to obtain individualpositive electrodes 1.

Also, using hardly (barely) graphitizable carbon as negative-electrodeactive material 7, negative-electrode active material 7 was formed oneach of the opposite surfaces of negative-electrode current collector 8.Negative-electrode active materials 7 according to the present example,as with first positive-electrode active material 2A, were configured soas to each include a flat portion having a thickness of 35 μm, athin-layer portion having a thickness of 5 μm and an inclined portionwhose thickness continuously decreases from the flat portion to thethin-layer portion. Then, the inclined portion and the thin-layerportion of each negative electrode 6 was disposed so as to face inclinedportion 2A₂ and thin-layer portion 2A₃ of relevant firstpositive-electrode active material layer 2A via a relevant separator.The rest of the conditions was made to be similar to those of example 1,and a stacked type secondary battery having a thickness of 3 mm wasobtained.

Comparative Example

A positive-electrode active material on each of the opposite surfaces ofpositive-electrode current collector 3 was formed as a layer having auniform thickness, the layer having neither a thin-layer portion, nor aninclined portion, nor a stepped portion, and was configured so as toconsist essentially of a flat portion with no inclined portion provided.The rest of the conditions was made to be similar to those of example 1,and a stacked type secondary battery was obtained. The thickness of thestacked type battery was approximately 8 mm at the center portion andapproximately 9 mm around an end portion.

<Evaluation>

To evaluate the discharge capacities and the cycle characteristics ofthe stacked type batteries obtained in the above manner, 10 stacked typebatteries for each of examples and comparative example were evaluated.It was found that the stacked type batteries according to examples 1 and2 provide a very stable discharge capacity and cycle characteristics,and that the discharge capacity and cycle characteristics of the batteryaccording to the comparative example are unstable compared to those ofthe batteries according to examples 1 and 2. The stable batterycharacteristics can be considered as resulting from preventing orreducing an increase in the thickness of a portion of the stacked typebattery in which insulating materials 40 are positioned from beingincreased so as to be larger than a thickness of the rest of theportions and thus enabling the stacked type battery to be held in placewhile uniform pressure is applied to it.

In each of the above examples, positive-electrode active materials 2 andnegative-electrode active materials 7 are formed by being intermittentlyapplied; however, as illustrated in FIGS. 15 to 17B, positive-electrodeactive materials 2 and negative-electrode active materials 7 may beformed by being continuously applied so as to form active material layerwith no intervals over a plurality of electrode forming portions. Whereactive materials are formed by means of being continuously applied,before the electrodes are cut out along each cutting line 90 in FIG.17A, they can be kept in the form of an electrode roll as illustrated inFIG. 18, and in such a case, extreme distortion of portions in whichinsulating materials 40 are disposed can be prevented, thereby enhancingthe product quality of the electrodes.

The present invention is useful for manufacturing electrodes for alithium ion secondary battery and manufacturing a lithium ion secondarybattery using such electrodes, and is also effectively employed for asecondary battery other than a lithium ion battery.

The present application claims priority from Japanese Patent ApplicationNo. 2013-166462 filed on Aug. 9, 2013, and the entire disclosure ofJapanese Patent Application No. 2013-166462 is incorporated herein byreference.

REFERENCE NUMERALS

-   1 positive electrode-   2 positive-electrode active material-   2A first positive-electrode active material layer-   2A₁ flat portion-   2A₂ inclined portion-   2A₃ thin-layer portion (small-thickness portion)-   2B second positive-electrode active material layer-   3 positive-electrode current collector-   4 boundary portion-   6 negative electrode-   7 negative-electrode active material-   8 negative-electrode current collector-   20 separator-   40 insulating material-   100 secondary battery

1. A secondary battery manufacturing method comprising: forming apositive electrode by applying a positive-electrode active material toeach of opposite surfaces of a positive-electrode current collector;forming a negative electrode by applying a negative-electrode activematerial to each of opposite surfaces of a negative-electrode currentcollector; and alternately stacking the positive electrode and thenegative electrode via a separator, wherein the forming of the positiveelectrode includes applying a positive-electrode active material layerto a surface of the positive-electrode current collector to form apositive-electrode active material layer consisting essentially of aflat portion, and then applying a positive-electrode active materiallayer to a surface on an opposite side of the surface with thepositive-electrode active material consisting essentially of a flatportion formed thereon to form a positive-electrode active materiallayer including a large-thickness portion and a small-thickness portionhaving a thickness that is smaller than that of the large-thicknessportion.
 2. The secondary battery manufacturing method according toclaim 1, further comprising disposing an insulating material to cover aboundary portion between a coated portion to which thepositive-electrode active material has been applied and a non-coatedportion to which the positive-electrode active material has not beenapplied in the positive electrode.